1. Field of the Invention
The present invention deals with rotary vane valves and, in particular, with means for limiting fluid leaks from such valves.
2. Description of the Prior Art
Rotary vane valves are used as airlock devices for transferring particulate solids between two regions or systems having different pressures or gas compositions. It is generally desirable that leakage be held to a minimum since such leakage can result in changes in pressure in either region or can constitute a hazard or create deleterious effects such as corrosion, contamination, erosion or systemic loss of efficiency.
Typical rotary vane valves consist of a cylindrical housing, with opposed inlet and outlet openings for material, and a rotating unit within the housing having a number of pockets for transferring material from inlet to outlet, much as a revolving door permits traffic to pass from a warm area to cold area with a minimum loss of heat. In certain rotary vane valves, the vaned rotor is fitted with annular side plates, called shrouds, which are welded to the ends of the vanes. The closer the clearances between housing and rotating unit can be held, the less leakage or loss of pressure there will be.
There are many reasons why the close clearances required are often impractical, costly, and, in certain cases, even impossible to achieve. Particulate material can be trapped or entrained between rotor and housing surface, causing bindings or arbrasion, or both. Temperature gradients between the housing and rotor, complemented by the fact that the masses of the two components may be significantly different, can result in differential amounts of thermal expansion and cause binding or seizure. It is also known that high pressure differentials can cause bending forces on the shaft which supports the rotor. If close clearances are used, the result is interference between the rotor and cylindrical housing. Additionally, as the physical dimensions of such rotary vane valve units increase the difficulty and cost of manufacture to close tolerances increases significantly.
In order to reduce leakage without the necessity of using such close tolerances, it has been suggested that the cylindrical housing be fitted on each of its ends with gas tight end bells, and the pressurized air or another gas be introduced into the space inside these end bells. This so-called "purge" gas is maintained at a pressure somewhat higher than the maximumgas pressure which acts on the rotor, and its purpose and effect is to prevent particulate matter being handled in the rotary valve from migrating across the rim of the rotor shrouds into the end bells. Not only would the particulate matter eventually fill up the end bells, but also it could cause severe friction and wear on the rims of the rotor shrouds and on the housing surfaces which face the shroud rims.
If there were no purge applied to the end bells, air or gas which the valve is intended to seal against would tend to flow across the rim of the shrouds into the end cavities, toward the region of lower pressure, i.e., from the discharge port to the inlet port in the case where the discharge port was exposed to greater pressure. At the inlet, the flow would be back across the shroud rims, again carrying particulate matter. The purge not only tends to keep the interface between rim of shroud and housing surface relatively free of particles, but also imposes a limitation on leakage.
The rate of air or gas flow, provided that the pressure ratio across the shroud is subcritical, Q(cfm) follows the theory of flow through orifices and is proportional to orifice area and to the square root of the differential pressure across the orifice. Considering that the orifice in this case is that formed by the clearance between the shrouds and the cylindrical housing, the relationship ##EQU1## is applicable regardless of which direction the gas flow takes. The use of an air or gas purge flow, it can be seen, not only minimizes friction and wear that can be caused by migrating particles, but also reduces leakage of the gas from the high pressure side to the low pressure side of the valve.
As the physical size of the rotary valve increases, the manufacturing and operating problems involving close clearances become more pronounced and more costly. The problems of leakage likewise become more significant and the amount of purging necessary to prevent such leakage will also increase. It will be appreciated that increases in purging requirements will also be necessitated as requirements for differential pressure capability grow greater, or as the temperature to which the valve is exposed becomes higher, or as a result of any combination of the above mentioned factors. It will also be appreciated that the purging of the ends bells is an energy-intensive procedure which may also involve a relatively heavy investment in purging equipment. Furthermore, since it is often necessary that a relatively inert fluid such as nitrogen or steam be used as the purging media instead of air, this procedure may also be relatively cost intensive.
In view, therefore, of these problems, my co-inventors and I, in our above referenced related application, suggested a rotary vane valve that was equipped with shrouds at the end of its vanes in which an annular throttle plate was positioned axially outwardly from at least one of the shrouds so that this annular throttle plate was not actually in contact with the shroud but was closely spaced from it. The outer peripheral edge of this annular throttle plate was adjacent the cylinder housing such that it either actually abutted that housing or so that there was only a small gap between this peripheral edge and the cylindrical housing. At the inner edge of the annular throttle plate a purge gas was introduced into the gap between the shroud and the throttle plate. This purge gas flowed first radially outwardly in this gap and then axially inwardly in the gap between the outer peripheral edge of the shroud and the cylindrical housing.
With regard to viscous flow between parallel plates, we noted that it had been known that pressure drop varies inversely as the cube of the spacing between the plates and directly with the length of the flow passage. Thus, due to the existence of relatively narrow gaps between the annular throttle plate and the shroud and the peripheral edge of shroud and the cylindrical housing, gas velocity was relatively high as was pressure drop between the inner edge of the annular throttle plate and the interior of the cylindrical housing.
We also noted that those skilled in the art would appreciate that from the known relationships applicable to viscous flow between parallel plates it would be possible to calculate the particular distance between the annular throttle plate and the shroud which would be preferred under a certain set of circumstances. Since the pressures inside the cylindrical housing are often unpredictable and variable, we also suggested that it would be preferable that a means be provided to adjust the distance between the shroud and the annular control plate.
We also suggested that the outwardly-facing surfaces of the moving rotor shrouds and/or the inwardly-facing surfaces of the stationary annular plates in our rotary vane valve could be provided with smooth surface finishes or could be artifically roughened by any suitable process to increase the frictional resistance to flow between the plates. It was also suggested that plate surfaces could be machined with intermeshing alternating ridge and groove configurations in their surfaces to prevent straight-line flow of gas in the gap, which effect would also increase total seal resistance to flow.
Although we provided a rotary vane valve in which leakage of process fluids and particulate material into the end bell areas was effectively controlled at a relatively low cost, I have observed that relatively greater amounts of purge gas may tend to flow between the shroud and the throttle plate at certain points along the peripheries of the shroud and the throttle plate as compared with other such peripheral points. In particular, when the inlet opening is at a lower pressure than the outlet opening, a greater amount of purge gas will tend to flow toward the inlet opening than toward the outlet opening. Similarly, when the outlet opening is at a lower pressure, a relatively greater amount of gas will flow toward the outlet opening. It is, therefore, the object of my present invention to provide a still further improved rotary vane valve which retains the advantages of our above described prior invention but which is also characterized by a substantially equal rate of flow of purge gas at all points along the periphery of the throttle plate.